1. FIELD OF THE INVENTION
The present invention relates to remote actuator devices and specifically to an actuator device which may be used to remotely select a gear range for a two speed transfer case.
2. DESCRIPTION OF THE RELATED ART
Presently two speed transfer case gear selection is accomplished by force applied through a sleeved cable connected to the transfer case gear range control lever. The force is typically applied through a driver operated lever located in the cab of the vehicle. Frictional resistance in the sleeved cable increases the force which the driver must apply to the lever to accomplish a shift of the transfer case. In large trucks the cable must be routed a great distance and often through a circuitous path. Consequently, a great deal of force is required to shift the transfer case through a sleeved cable.
Remote actuator devices are available which derive motive force from compressed air, hydraulic fluid or electrical energy. Simple actuation devices utilizing these sources of motive force are subject to failure upon interruption of the source of motive force. Such a failure manifests itself as a transfer case which is stuck in the gear range previously selected. The previous gear selection is only by coincidence that which is appropriate for limp-home operation. Therefore, there exists a need to provide a stand-by supplemental motive force which will shift the transfer case to a limp-home gear range in the event of interruption of the source of motive force. Return springs are known in the art for achieving this purpose. However, return springs represent an additional force which must be overcome when shifting the transfer case.
The force required to shift the transfer case from HIGH or LOW into NEUTRAL involves a force which must disengage a jaw clutch which is accomplished by moving the clutch axially along a splined shaft. This force requirement peaks at the early portion of the stroke and then drops to a minimum. Conversely, the force required to shift the transfer case to HIGH or LOW from NEUTRAL involves a force which must engage a jaw clutch with a mating gear. This force requirement peaks as the jaw clutch engages the gear and accomplishes synchronization.
These force peaks define minimums necessary to accomplish the shift. In each respective instance, either shifting into engagement or out of engagement, the force peaks are approximately equal. If insufficient force is applied the shift will not occur. If too much force is applied during disengagement, the shift will be prevented due to binding of the engagement splines. In addition, too much force will result in an abrupt shift during engagement risking damage to the respective gears.
An actuator must be designed taking into account the presence of the stored spring force. Thus an actuator must provide a smaller amount of force in the early portion of the stroke and progressively more as the actuator reaches the end of its stroke. Therefore, there exists a need for a simple actuator which can achieve three position control of a transfer case which provides a fail-safe, limp-home gear change while providing equal amounts of force to accomplish respective shifts either into engagement or out of engagement.